It's almost funny, actually...one of the cars has been sitting for about a year - heated indoor shop - and the only way to select gears in the A833 now is engine off. Clutch does not release. It'll start in gear, though.
Linkage fine, nothing was changed in that time except a starter replaced, pedal moves normally.
I'm thinking the disc is stuck on the flywheel or pressure plate or both; and the only way I can think of so far to free the disc is to warm the car up on stands & in gear and then shock it loose with the brakes.
Anybody have that happen before?

Its happened to other people I know, and you're right about how to shock it loose

Make sure the pedal is down when you hit the brakes

Happened to me for the first time about a year ago. Car was sitting a month or so. I couldn't believe it was the issue but everything else checked out. Pulled inspection cover and with petal to floor there was plenty of gap between pressure plate and disc. Took small flat blade screwdriver and loosened disc from flywheel. Didn't take much to loosen. I've had manual trans cars for 30+ years and this was the first.

Your idea will prob work also.

Unfortunately for this problem, the car has a Lakewood bell on it, so there's no access unless I take it all apart. I'd love right now to be able to just pry the damn disc loose...

Take it out on the street with plenty of open road ahead, put it into 1st, depress the clutch pedal, engage the starter....with any luck it'll break loose with the starter alone but, if not, let it start then, with the clutch pedal still depressed, pump the throttle...that should break the disc loose.

Crank it and let it start in gear with the clutch in as soon as it starts it will break lose!

Yep, my old 67 Hemi did that once (lived in humid Florida at the time). Do as instructed, point it towards and area that is safe to travel quickly/unexpectedly and let her rip, it'll come loose.

Thanks for the replies.
Can't really drive it where it sits, so I'm gonna try it with the rearend up on jackstands. Tried starting it in gear and moving about a foot but it didn't free up. If I move the other cars out of the way, I can get 20-25 feet.

Disc is stuck (rusted) to the flywheel. This can happen in a matter of days!! I installed a brand new clutch to a brand new flywheel and then bagged the whole motor in a dry environment and the disc still rusted to the flywheel - it had only been on a couple weeks. I think the disc absorbs a bit of moisture in storage and the pressure of the pressure plate squeezes it out and causes the issue.